In recent years, white organic EL devices have been developed positively because they are usable for a mono-color display device, lighting such as backlighting, a full-color display device using a color filter and so on. Chromaticity change in a white organic EL device degrades its quality as a product, and furthermore it causes poor color reproducibility, for example, in a full-color display combined with a color filter. A white organic EL device with a small chromaticity change is thus desired.
There have been disclosed a number of methods to achieve white emission using organic EL materials. Few of the methods produce a white color using only one kind of emitting material; usually 2 to 3 kinds of emitting materials emit at the same time in single organic EL device. In case where three kinds of emitting materials are used, the combination of red, blue and green emission, which corresponds to the three primary colors, produce a white color. However, there is a problem that chromaticity control is difficult and the reproducibility is poor.
In case where two kinds of emitting materials are used, a blue emitting material and a yellow-to-red light emitting material, yellow-to-red being the complementary color of blue, are selected. However, the emission of yellow-to-red is often intensified to easily cause a change in color. In conventional white organic EL devices, for example, as shown in Reference Examples 1 and 2 of JP-A-2001-52870, blue light tends to easily weaken with a color change.
A white emission can also be obtained by doping with a blue dopant and a yellow-to-red dopant at the same time and adjusting their doping ratio. However red trends to intensify and, furthermore, energy easily transfers from blue to red, thereby yielding a white color tinged with red. In order to obtain a white color, it is necessary to use a yellow-to-red dopant at a very low concentration and reproducibility is therefore difficult to achieve.
There is a method in which a yellow-to-red material is doped into a hole-transporting layer adjacent to an emitting layer. In this method, since it is difficult to inject electrons into the hole-transporting layer, a red light is not strongly emitted, even though the yellow-to-red dopant tends to emit intensified light. Therefore, it is easy to balance blue emission and yellow-to-red emission to attain white emission. The method has an excellent luminous efficiency and long lifetime. However, it has a serious problem in that the chromaticity change is large after continuously driving or storage at high temperatures, which problem is caused by the distance dependency of energy transportation.
The inventors have found that the reason for chromaticity change is probably as follows: excited molecules for red emission are concentrated in an interface on the hole-transporting layer side. Balance between electrons and holes is lost by degeneration. Consequently even a small change in degree of concentration in the interface causes a large change in red emission but no large change in blue emission.
The type that divides an emitting layer into two layers includes the stacked type where an emitting layer near an anode is a yellow-to-red emitting layer, while an emitting layer near a cathode is a blue emitting layer. This type is excellent in efficiency. However, in order to obtain a white color, the thickness of the yellow-to-red emitting layer must be thinner than that of the blue emitting layer, or the concentration of a dopant in the yellow-to-red emitting layer must be smaller than that in the blue emitting layer to suppress yellow-to-red emission. Consequently fabricating the device is difficult. Specifically the thickness of the yellow-to-red emitting layer is often required to be about 1 to 2 nm for white emission. This thickness is as thin as the molecule size of ordinary low molecule type organic EL materials and controlling the thickness is thus extremely difficult.
On the other hand, by making an emitting layer on the anode side, toward which the emission region is liable to shift, a blue emitting layer, the tendency of the emitted light color to shift to red is counteracted, white light emission can be obtained and chromaticity change during driving is less, even when the yellow-to-red emitting layer is given a thickness of approximately 10 to 30 nm. However, in view of practical use, a stable white organic EL device is desired whose chromaticity change is even smaller.
Recently a white emission at a high efficiency is realized by using two phosphorescent emitting layers and arranging an exciton-blocking layer therebetween (refer to Applied Physics Letters, 83, 2459 (2003)). The exciton-blocking layer helps to block holes, but it has a large affinity level so that it traps electrons with an increased driving voltage of the device. It is thus required to use a hole-injecting material such as PEDOT.PSS (poly(3,4-ethylenedioxythiophen)/poly(styrenesulfonic acid)) which enables significantly low voltage.
An object of the invention is to provide an organic EL device which is small in chromaticity change and has high efficiency.